Gas lasers are used as light sources for material machining, such as marking on products, and boring, cutting, and modification of material. In particular, rare-gas halogen excimer lasers are used as light sources for marking on organic material, ablation processing, microfabrication of general material, surface modification, photochemical reaction, and the like. Such rare-gas halogen excimer lasers are also used in the processor manufacturing semiconductor products. For example, they can be used as light sources of projection aligners used in optical lithography for forming circuit patterns on semiconductors. Further, fluorine excimer lasers, which have mechanisms comparable with those of the rare-gas halogen excimer lasers and are provided for generating ultraviolet rays shorter in wavelength than those from the rare-gas halogen excimer lasers, are expected to be used in the same fields as the rare-gas halogen excimer lasers are used. Hereinbelow, the rare-gas halogen excimer lasers and the fluorine excimer lasers are generically called the excimer lasers.
FIG. 15 is a perspective view illustrating the structure of a typical conventional discharge-pumped gas laser. The following description is made based on the drawing. A laser chamber 1 is a container filled with laser medium gas (hereinafter, referred to as laser gas) for oscillating laser light. Provided inside the laser chamber 1 are a main discharge electrode 2, for performing glow discharge to excite or pump the laser gas, and a preionization electrode 3, for performing preionized discharge to generate initial electrons in a main discharge space of the main discharge electrode 2. Also provided inside the laser chamber 1 are a fan 7, for circulating the laser gas to supply laser gas into the main discharge space, and a heat exchanger 8, for cooling the laser gas whose temperature has risen due to discharge energy. Provided outside the laser chamber 1 is a high-voltage pulsed power source 4 for supplying the main discharge electrode 2 and the preionization electrode 3 with discharge energy. A capacitor is provided inside the high-voltage pulsed power source 4 for storing the discharge energy. The discharge energy is controlled by controlling a charging voltage across the capacitor. The laser gas used can be a mixture of carbon dioxide gas, helium gas and nitrogen gas in case of carbon dioxide gas lasers; a mixture gas of fluorine, krypton, and a buffer gas (helium or neon) in case of Kr F excimer lasers; and a mixture of fluorine, argon, and a buffer gas (helium or neon) in case of Ar F excimer lasers.
In typical gas lasers, a component gas showing relatively high reactiveness, e.g., a fluorine gas, reacts while adhering to the inside of the laser chamber 1 or to the surfaces of the fan 7 and the heat exchanger 8. The component gas also reacts while being adsorbed onto the surfaces of metal particles caused by sputtering of electrode material at the time of discharge-pumping. Thus, the concentration of the component gas showing relatively high reactiveness decreases with time. On the other hand, the component gas reacts with moisture in the laser chamber 1, a hydrogen atom occluded into the metal or a hydrogen atom in a high polymer such as a seal material or lubricant used in the laser chamber 1, to generate an impurity gas, resulting in an increase in the concentration of the impurity gas.
When the concentration of the component gas in the laser gas decreases to a prescribed value or less, or the concentration of the impurity gas increases to a prescribed value or more, the laser output power is reduced. In many cases, applications for the gas laser require that the laser output power be kept constant. In general, in order to keep the laser output power constant, the gas concentration is controlled by injecting a proper amount of component gas having relatively high reactiveness into the laser gas, or a charging voltage of the high-voltage pulsed power source 4 is controlled so that the power-on energy for pumping will increase.
However, the increase in the charging voltage is restricted due to a limited pressure resistance of the laser chamber 1 or such limited energy that the high-voltage pulsed power source 4 can charge. It is also necessary to maintain laser output characteristics (e.g., the width of spectral line or the beam width) other than the laser output power, and this restricts the injected amount of component gas. In other words, there is a limit to the control of the laser output power by the adjustment of the injected amount of component gas or the power-on energy. A laser gas, which no longer has the ability to control the laser output power, is called the "used laser gas" hereinbelow. When the control of the laser output power approaches the limit, the laser gas is generally replaced with fresh laser gas. The process to replace the laser gas can be performed as follows. The used laser gas in the laser chamber 1 is exhausted through an exhauster, not shown, so that the laser chamber 1 becomes a substantial vacuum. After that, a prescribed amount of fresh laser gas is injected into the laser chamber 1 from laser gas supplying means such as a laser gas cylinder, not shown. This makes it possible to restart the control of the laser output power, and hence to expect a constant output.
When all of the used laser gas is replaced with fresh laser gas, the gas laser is theoretically expected to make a high-level recovery of the laser output power from the start of laser oscillation immediately after the fresh laser gas is charged, but the laser output power immediately after the fresh laser gas is charged is actually reduced. Such a reduction in the laser output power takes place remarkably in excimer lasers, and is much more remarkable in Ar F excimer lasers and fluorine excimer lasers.
FIG. 16 illustrates a time transition of the laser output power from the time immediately after the fresh laser gas is charged. This case assumes that excitation or pumping energy is kept constant without any control to maintain a constant output power such as injection of component gas. The output power has a tendency to start from a low value immediately after the fresh laser gas is charged, then gradually increase, and reach a normal value after a prescribed time has elapsed. Since the output power is very low immediately after the fresh laser gas is charged, a prescribed rated output power cannot be outputted even when the pumping energy is increased up to an upper limit. To solve such a problem, several methods, such as ones shown below, have been adopted without understanding why those methods are effective.
A first method is to leave the laser without oscillation for a period of about 10 minutes to one hour after the fresh laser gas is charged. When the laser starts oscillating after leaving it inactive, a prescribed rated output power can be obtained from the start of laser oscillation.
A second method is to perform laser oscillation for a period of about several minutes to one-half hour after the fresh laser gas is charged. In this case, the prescribed rated output power can be obtained earlier than the first method.
A third method is to increase the temperature of the fresh laser gas to a value between about 30 and 40 C. .degree.. In this case, the prescribed rated output power can also be obtained earlier than the first method.
However, the first method for leaving the laser without oscillation after the fresh laser gas is charged causes a decreased operating ratio of the laser. Gas replacement generally takes 15 minutes or so, and the laser is required to stop oscillation for more than one hour maximum including the time required for gas replacement. This can cause a problem in that the laser reduces its production efficiency.
The second method, though able to reduce the time compared to the first method, also causes the laser to reduce its production efficiency due to an increased idle time of the laser. Further, energy consumed by laser oscillation for output power recovery is not directed to production, and such energy consumption is not preferable for conservation of energy. Besides, it can decrease the life of the laser and the laser gas.
Since the third method is to control the temperature of the laser gas in an optimum range, there is a need to adjust the flow rate of a refrigerant flowing through the heat exchanger 8, or to provide a laser gas temperature adjusting mechanism for adjusting the temperature of the refrigerant. This makes the entire laser mechanism complicated, resulting in decreased reliability.